I. Field of the Invention
The present invention relates to a method for removing flashes formed in a molded resin product such as a plastic encapsulated semiconductor package.
II. Description of the Prior Art
In a plastic encapsulated semiconductor package 1 shown in FIG. 1, a semiconductor element (not shown) such as an IC or an LSI is placed on a lead frame 2. After wiring is formed, the structure is placed in a mold, and an epoxy resin is injected into the mold for encapsulation. However, during molding, the epoxy resin leaks through the gap between the lead frame 2 and the mold to form flashes 3 on the lead frame 2 and in the gaps between the leads of the lead frame 2. When a columnar electric element such as a diode is to be plastic-encapsulated, the epoxy resin leaks out through the gap between a pair of upper and lower molds, thereby forming flashes around the columnar molded product. Plastic encapsulation of such a columnar electric element therefore requires a post-molding step to remove such flashes. In view of this problem, a hard abrasive such as alumina, silicon carbide or glass bead, or a soft abrasive such as a walnut shell powder is blasted onto the flashes 3 to remove them.
However, a problem is encountered when a hard abrasive is used. That is, an abrasive with a hardness H.sub.R C of 70 or more while the epoxy resin useful for a semiconductor device has a hardness H.sub.R M of about 100. Thus, the abrasive is far harder than the epoxy resin, and has a specific gravity four times that of the epoxy resin. For this reason, when the flashes 3 are removed, the surface of the package 1 is damaged, to have a poor outer appearance. In addition to this, moisture may be introduced through the damaged portions of the package 1, adversely affecting reliability of the semiconductor element. On the other hand, a problem is also encountered when a soft abrasive such as a walnut shell powder is used. Since such a soft abrasive has an removal ability which is weaker than that of a hard abrasive, a soft abrasive must be blasted at a higher pressure than a hard abrasive. This leads to deformation of the lead frame 2 and a higher running cost of the equipment for manufacturing the molded products, since a larger amount of compressed air is used. Furthermore, when a soft abrasive is used, static electricity is generated between the abrasive and the molded product 1 upon contact therebetween. The static electricity firmly attaches the fine powder of the abrasive, flashes and the like to the surface of the package 1. As a result, an outer appearance of the package 1 is degraded in a subsequent soldering or plating step. This also leads to a problem of corrosion of the lead frame 2.